The present invention relates a method of forming a structure comprising a removed layer of semiconductor material taken from a donor wafer, the donor wafer comprising, prior to removal, a first layer formed of Si1-xGex and a layer of Si1-yGey on the first layer, with x and y respectively being in the range of 0 to 1 and x being different from y. This method comprises the following steps in succession:
a) implanting atomic species to form a zone of weakness beneath the second layer;
b) bonding the donor wafer to a receiver wafer;
c) supplying thermal and/or mechanical energy to detach the removed layer from the donor wafer at the zone of weakness;
d) treating the removed layer.
The preferred type of layer removal uses a SMART-CUT® technique, a layer transfer method that is well known to a skilled person. One example of employing such a removal method is described in U.S. patent publication 2004/0053477 in which the crystallographic structure of the second layer is elastically strained by the structure of the first layer.
Step d) for treating the removed layers often has to be carried out to lift off defective zones and to reduce surface roughness principally resulting from carrying out steps a) and c). The thickness of the defective zone is typically about 150 nanometers (nm) for atomic implantation of hydrogen ions. As an example, it is possible to perform mechanical polishing or chemical-mechanical planarization (CMP, chemical-mechanical polishing) in order to eliminate surface roughness, and/or steps of sacrificially oxidizing defective zones.
Since bonding in accordance with step b) is conventionally carried out via a layer of electrically insulating material, a semiconductor-on-insulator structure can thus be produced, such as a Si1-xGex/Si1-yGey on insulator structure. As disclosed in U.S. patent publication 2004/0053477, a step subsequent to step d) can be carried out to lift off the remaining portion of the first layer, hence retaining only the second layer on the receiver wafer. Thus, a Si1-xGex on insulator structure can be produced.
The operation for lifting off the remaining portion of the first layer can be carried out effectively by selective chemical etching using suitable etching agents. Selective chemical etching can in the end produce the desired layer with a good surface quality without too great a risk of damaging it (which could be the case if, e.g., only a single polish was to be carried out). But selective chemical etching necessitates prior preparation of the etching surface, typically carried out using mechanical polishing means. That preparation step remains necessary to reduce the severe roughness which could subsequently cause locally over-inhomogeneous etching, which in turn could create through defects or holes in the second layer. The successive actions of polishing and chemical etching, however, render the post-detachment finishing step (as well as the whole of the removal procedure) long, complex, and expensive.
Furthermore, chemical etching can in some cases result in problems with at least partial unbonding or disbonding of the bonding interface. In particular, it may delaminate an edge or side of the bonding layer, i.e., attack the layer where it crops out from the side of the resulting structure. An example that can be mentioned is hydrofluoric acid (“HF”) treatment of a sSOI (strained silicon on insulator) structure comprising SiO2 buried under strained Si, or H2O2:HF:HAc treatment (HAc being the abbreviation for acetic acid) on a sSi/SiGeOI (strained silicon on SiGe on insulator) structure, where the layers of SiGe and buried SiO2 are liable to be etched beneath the layer of strained Si. Thus as regards the quality of the final product, the results obtained are less than satisfactory.
One alternative which could be envisaged to overcome that problem would be to dilute the etching agents further to enable their action to be better controlled, but this requires a longer procedure to etch the surface due to the use of dilute acids. That solution is not satisfactory since by substantially increasing the duration of the procedure, it still does not completely solve the problem of delamination at the edges or sides of the resulting structure.
A further solution which could be envisaged would be to reinforce the bonding interface prior to etching to render that interface more resistant to chemical agents. To this end, a post-detachment stabilizing heat treatment carried out at about 1000° C. or more for several hours could be envisaged, but that solution, which is well known when producing a SOI (silicon on insulator) structure, is not suitable when transferring heterogeneous layers of Si1-xGex and Si1-yGey. In fact, such a heat treatment causes germanium to diffuse from the layer having the highest Ge content towards the layer having the lowest Ge content, thus tending to homogenize the Ge content over the two layers so that the physical and electrical properties of those two layers can no longer be differentiated. If the two layers become essentially identical, then subsequent etching can no longer be selective and it is not possible to remove only the first layer without removing part of the second layer.
Further, it is frequently desirable to avoid any diffusion from one layer to another. This is particularly the case when the second layer is formed of strained Si (i.e., Si1-yGey where y is 0) and when a final sSOI (strained SOI) structure is to be obtained to fully benefit from the electrical properties of the structure (e.g., increased charge mobility). Thus, the treatment temperature is limited by the diffusion of germanium from one layer to the other (with the diffusion typically commencing at about 800° C.), and low temperature reinforcement can thus only be partially effective. Thus, the delamination problem persists even with such reinforcement.
The present invention now seeks to avoid these problems by providing a new process for treating layers to be removed in a manner that does not detrimentally affect the remainder of the structure.